1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal processing method for converting multi-channel digital audio signals Into analog audio signals and outputting the analog audio signals; a program for executing signal processing; and a recording medium used for recording the program.
2. Description of the Related Art
A conventional signal processing apparatus 300 for converting multi-channel digital audio signals into analog signals and outputting the analog signals will be described with reference to FIGS. 9, 10 and 11. The signal processing apparatus 300 is incorporated in, for example, a DVD-Video player. The DVD-Video standards support reproduction of multi-channel audio signals up to 5.1 channels. FIG. 10 shows the arrangement of a 5.1 channel speaker unit. 5.1 channels means, as shown in FIG. 10, 5 channels including a left forward (L: left) center forward (C: center), right forward (R: right), left surround (LS), and right surround (RS) channel, and one channel of a low frequency effect channel (LFE).
FIG. 9 shows a structure of the signal processing apparatus 300. According to the DVD-Video standards, a 5.1 channel audio bit stream signal 40 is input to the signal processing apparatus 300. A decoder 6 receives the audio bit stream signal 40 and decodes the audio bit stream signal 40 into a digital audio signal (linear PCM). Then, the audio bit stream signal 40 separates the digital audio signal into a digital audio signal 31 of a first channel (a first channel digital audio signal 31), a digital audio signal 32 of a second channel (a second channel digital audio signal 32), . . . a digital audio signal 3n of an n'th (n≧2) channel (an n'th channel digital audio signal 3n), and a digital audio signal 30 of an LFE channel (an LFE channel digital audio signal 30). In the case of the 5.1 channel system, n=5. A down-mixing signal processing section 3 receives resultant digital audio signals 30, 31, . . . 3n and performs down-mixing signal processing.
Down-mixing signal processing can be performed in various manners. In the case of the 5.1 channel system, down-mixing signal processing can be performed, for example, as shown in FIG. 11. The down-mixing signal processing section 3 down-mixes the digital audio signals of 5.1 channels of L, R, C, SL, SR and LFE channels to 2.1 channels of L, R and LFE. In FIG. 11, digital audio signals of the SL, L, C, R, SR and LFE channels are indicated by reference numerals 51, 52, 53, 54, 55 and 50. The down-mixing signal processing section 3 includes multipliers 8a, 8b, 8c, 8d, 8e and 8f and adders 9a and 9b. Multiplication coefficients of the multipliers 8a, 8b, 8c, 8d, 8e and 8f are respectively m1, m2, m3, m4, m5 and m6. The multiplier 8a multiplies the SL channel digital audio signal 51 with the multiplication coefficient m1. The multiplier 8b multiplies the L channel digital audio signal 52 with the multiplication coefficient m2. The multiplier 8c multiplies the C channel digital audio signal 53 with the multiplication coefficient m3. The multiplier 8d multiplies the R channel digital audio signal 54 with the multiplication coefficient m4. The multiplier 8e multiplies the SR channel digital audio signal 55 with the multiplication coefficient m5. The multiplier 8f multiplies the LFE channel digital audio signal 50 with the multiplication coefficient m6. The digital audio signals 51 through 55 and 50 respectively correspond to the digital audio signals 31, 32, . . . 3n and 30 shown in FIG. 9.
The adder 9a adds output signals from the multipliers 8a, 8b and 8c, and outputs a digital audio signal 56 of an L channel (an L channel digital audio signal 56). The adder 9b adds output signals from the multipliers 8c, 8d and 8e, and outputs a digital audio signal 57 of an R channel (an R channel digital audio signal 57). The multiplier 8f outputs a digital audio signal 58 of an LFE channel (an LFE channel digital audio signal 58).
An exemplary general ratio of the multiplication coefficients is m1:m2:m3:m4:m5:m6 0.7:1.0:0.7:1.0:0.7:1.0. The ratio of the multiplication coefficients is changeable in accordance with characteristics of the input signal or the system. In the case where a signal which is to be input to the down-mixing signal processing section 3 is level-adjusted so as to avoid an overflow, the ratio of the multiplication coefficients can be the above-mentioned ratio. In the case where there is a possibility that down-mixing signal processing causes an overflow, the multiplication coefficients m1 through m6 need to be regulated in advance. In the case where the LFE, SL, L, C, R and SR channel digital audio signals 50, 51, 52, 53, 54 and 55 are not processed against an overflow, all the multiplication coefficients m1 through m6 further need to be regulated with 1/(2.4).
The L, R and LFE channel digital audio signals 56, 57 and 58 obtained by down-mixing signal processing are given to D/A converters 63, 64 and 65 shown in FIG. 9. The D/A converter 63 converts the L channel digital audio signal 56 into an analog audio signal 56′ of an L channel (an L channel analog audio signal 56′) and outputs the L channel analog audio signal 56′. The D/A converter 64 converts the R channel digital audio signal 57 into an analog audio signal 57′ of an R channel (an R channel analog audio signal 57′) and outputs the R channel analog audio signal 57′. The D/A converter 65 converts the LFE channel digital audio signal 58 into an analog audio signal 58′ of an LFE channel (an LFE channel analog audio signal 58′) and outputs the LFE channel analog audio signal 58′.
One D/A converter is required for each channel. Therefore, the signal processing apparatus 300 shown in FIG. 9 requires three D/A converters 63, 64 and 65. In most of the actual products, however, two D/A converters are packaged into one LSI. Where two such LSIs are incorporated into the signal processing apparatus 300, one D/A converter is not used. In addition, the D/A converters used for DVD players are mostly expensive in order to provide high quality sound.
When a user reproduces video or audio data using a DVD player, he/she often uses a speaker unit which is not of a surround system. Often times, he/she does not use the LFE channel. In a portable DVD player, a headphone speaker is often used for outputting the audio data, in which case, the LFE channel is not used. Furthermore, the output from the DVD player is often reproduced by a general TV receiver. A speaker unit of most of the TV receivers have only an L channel and an R channel and is not of a surround system. The LFE channel is not used.
In the conventional signal processing apparatus, one D/A converter is provided for each channel for converting a digital signal into an analog signal although often times the LFE channel is not used. In the case of a 2.1 channel output system, three D/A converters are required, which unnecessarily increases the cost.